Notwithstanding the significant advances which have been made over the years in respect of materials capable of delivering improved metallurgical properties, considerable research efforts continue in the search for new alloys to satisfy the demands of advanced design in the aircraft, automotive and electrical industries. While high strength is a key characteristic of the materials sught, to meet the qualifications for certain advanced design applications, the alloys must meet a combination of property requirements such as density, ductility, fracture toughness, corrosion resistance as well as strength, depending on the ultimate end use of the materials.
Aluminum-lithium alloys are potential candidates for many applications when low density and high elastic modulus are important. The present invention applies to aluminum-lithium alloys containing a dispersoid constituent, as will be described more fully below.
Heretofore, many aluminum-lithium alloy systems made by ingot and powder metallurgy routes have been studied. Efforts have been made to strengthen the systems by incorporating additives in the alloy to cause or increase precipitation hardening or to distribute a dispersoid in the alloy. While effective, there are limits to the amount of strengthening agents that can be added without sacrificing other properties such as ductility, fracture toughness and corrosion resistance. Certain alloys can be aged to increase strength. However, even in the aged condition, the alloys can not meet the desired combination of target properties specified.
The complexity of the problem goes far beyond the difficulties of developing materials with suitable combinations of properties not achieved before. Economics also plays a large role in the choice of materials. The ultimate product forms are often complex shapes, and the potential savings resulting from possible composition substitution is only a part of the picture. The new aluminum alloys would be particularly valuable if they could be shaped into desired forms using cost effective techniques such as forging while retaining their preshaped properties and/or if they could be fabricated economically into the same complex shapes now used with other materials so as to eliminate the need for retooling for fabrication of weight saving structures. Moreover, to be commercially useful, the fabricated parts must have reproducible properties. From a vantage point of commercial viability, the reproducibility will be attainable under a practical range of conditions.
The present invention is not confined to any one route known in the art for producing the alloy products. It can be incorporated into the process subsequent to the shaping steps, as will be further described below. However, it is particularly useful further when incorporated into a powder metallurgy route, and it is especially useful in the preparation of aluminum-lithium alloys from mechanically alloyed powder.
The use of powder metallurgy routes to produce high strength aluminum has been proposed and has been the subject of considerable research. Powder metallurgy techniques generally offer a way to produce homogenous materials, to control chemical composition and to incorporate dispersion strengthening particles into the alloy. Also, difficult-to-handle alloying elements can at times be more easily introduced by powder metallurgy than ingot melt techniques. The preparation of dispersion strengthened powders having improved properties by a powder metallurgy technique known as mechanical alloying has been disclosed, e.g., in U.S. Pat. No. 3,591,362 (incorporated herein by reference). Mechanically alloyed aluminum-base alloys are characterized by fine grain structure which is stabilized by uniformly distributed dispersoid particles such as oxides and/or carbides. U.S. Pat. Nos. 3,740,210 and 3,816,080 (incorporated herein by reference) pertain particularly to the preparation of mechanically alloyed dispersion strengthened aluminum. Other aspects of mechanically alloyed aluminum-base alloys have been disclosed in U.S. Pat. Nos. 4,292,079, 4,297,136 and 4,409,038.
It is academic that composition of an alloy often dictates the fabrication techniques that can be used to manufacture a particular product. In general, the target properties which must be attained in the type aluminum alloys of this invention before other properties will be considered are strength, density and ductility. One of the marked advantages of dispersion strengthened mechanically alloyed powders is that they can be made into materials having the same strength and ductility as materials made of similar compositions made by other routes, but with a lower level of dispersoid. This enables the production of alloys which can be fabricated more easily without resorting to age hardening additives. While the mechanical alloying route produces materials that are easier to fabricate than other aluminum alloys of comparable composition, the demands for strength and low density and the additives used to obtain higher strength and/or lower density usually decrease workability of the alloy system. (Workability takes into account at least ductility at the working temperature and the load necessary to form the material.) The extent of the effect is generally related to the level of additive in the alloy. The additives not only affect the method by which the material can be fabricated, but also the fabrication techniques affect the properties of the materials.
For most uses a powder must be fabricated into a final product, e.g., by degassing, compaction, consolidation and shaping in one or more steps. To obtain complex parts the fabrication may take the form, e.g., of extruding, forging and machining. Usually, the less machining required to make a part the greater the economy in material use, labor and time. It will be appreciated that it is an advantage to be able to make a complex shape by forging rather than by a route which requires the shaping by manual labor on an individual basis.
U.S. patent application No. 664,058, filed Oct. 23, 1984, discloses a method for producing low density, dispersion strengthened aluminum-lithium alloys into forged parts characterized by improved strength by shaping, i.e. extruding and forging, the alloys under certain conditions. The disclosed method to produce forged parts carries with it the advantages of using a powder metallurgy route, mechanical alloying and forging, as explained above. The present invention will be illustrated below mainly with reference to the method of such application, which is incorporated herein by reference.
It was unexpected that heat treatment for improving fracture toughness could be carried out without reducing tensile properties in the non-aged condition. It was particularly surprising that forged material is amenable to such treatment because the temperatures for heat treatment according to this invention are found to have an adverse effect on strength if used during forging.